Non-destructive testing system

ABSTRACT

A non-invasive testing system.

This application claims the benefit of U.S. Provisional Application No.60/377,419, filed Dec. 6, 2001.

BACKGROUND OF THE INVENTION

The present invention relates to non-invasive testing.

The development of advanced integrated circuit devices and architectureshas been spurred by the ever increasing need for speed. For example,microwave, fiber optical digital data transmission, high-speed dataacquisition, and the constant push for faster digital logic in highspeed computers and signal processors has created new demands onhigh-speed electronic instrumentation for testing purposes.

Conventional test instruments primarily include two features, theintegrated circuit probe that connects the test instrument to thecircuit and the test instrument itself. The integrated circuit probe hasits own intrinsic bandwidth that may impose limits on the bandwidthachievable. In addition, the probe also determines an instrument'sability to probe the integrated circuit due to its size (limiting itsspatial resolution) and influence on circuit performance (loading of thecircuit from its characteristic and parasitic impedances.). The testinstrument sets the available bandwidth given perfect integrated circuitprobes or packaged circuits, and defines the type of electric test, suchas measuring time or frequency response.

Connection to a test instrument begins with the external connectors,such as the 50 ohm coaxial Kelvin cable connectors (or APC-2.4). Theintegrated circuit probes provide the transitions from the coaxial cableto some type of contact point with a size comparable to an integratedcircuit bond pad. Low-frequency signals are often connected with needleprobes. At frequencies greater than several hundred megahertz theseprobes having increasing parasitic impedances, principally due to shuntcapacitance from fringing fields and series inductance from long, thinneedles. The parasitic impedances and the relatively large probe sizecompared to integrated circuit interconnects limit their effective useto low-frequency external input or output circuit responses at the bondpads.

Therefore, electrical probes suffer from a measurement dilemma. Goodhigh-frequency probes use transmission lines to control the lineimpedance from the coaxial transition to the integrated circuit bond padto reduce parasitic impedances. The low characteristic impedance of suchlines limits their use to input/output connections. High-impedanceprobes suitable for probing intermediate circuit nodes have significantparasitic impedances at microwave frequencies, severely perturbing thecircuit operation and affecting the measurement accuracy. In both cases,the probe size is large compared to integrated circuit interconnectsize, limiting their use to test points the size of bond pads. Likewisesampling oscilloscopes, spectrum analyzers, and network analyzers relyon connectors and integrated circuit probes, limiting their ability toprobe an integrated circuit to its external response. For networkanalysis, a further issue is de-embedding the device parameters from theconnector and circuit fixture response, a task which grows progressivelymore difficult at increasing frequencies.

With the objective of either increased bandwidth or internal integratedcircuit testing with high spatial resolution (or both) differenttechniques have been introduced. Scanning electron microscopes or E-beamprobing uses an electron beam to stimulate secondary electron emissionfrom surface metallization. The detected signal is small for integratedcircuits voltage levels. The system's time resolution is set by gatingthe E-beam from the thermionic cathodes of standard SEM's. Fordecreasing the electron beam duration required for increased timeresolution, the average beam current decreases, degrading measurementsensitivity and limiting practical systems to a time resolution ofseveral hundred picoseconds. Also, SEM testing is complex and relativelyexpensive.

Valdmanis et al., in a paper entitled “Picosecond Electronics andOptoelectronics”, New York: Springer-Verlag, 1987, shows anelectro-optic sampling technique which uses an electrooptic lightmodulator to intensity modulate a probe beam in proportion to a circuitvoltage. Referring to FIG. 1, an integrated circuit 10 includes bondedelectrical conductors 12 fabricated thereon whereby imposingdifferential voltages thereon gives rise to an electric field 14. Forcarrying out a measurement an electro-opti needle probe 16 includes anelectro-optic tip 18 (LiTaO₃) and a fused silica support 20. A lightbeam incident along path 22 is reflected at the end of the electro-optictip 18 and then passes back along path 24. An electric field 14 altersthe refractive index of the electro-optic tip 18 and thereby alters thepolarization of the reflected light beam on the exit path 24, which thusprovides a measure of the voltages on the conductors 12 at a singlepoint. Unfortunately, because of the proximity of the probe 16 to thesubstrate 10 capacitive loading is applied to the circuit, therebyaltering measurements therefrom. In addition, it is difficult toposition the probe 16 in relation to the conductor because the probe 16and circuit 10 are vibration sensitive. Also, the measurements arelimited to conductors 12 on or near the surface of the circuit 10.Further, the circuit must be active to obtain meaningful results and thesystem infers what is occurring in other portions of the circuit by alocal measurement.

Weingarten et al. in a paper entitled, “Picosecond Optical Sampling ofGaAs Integrated Circuits”, IEEE Journal of Quantum Electronics, Vol. 24,No. 2, February 1988, disclosed an electro-optic sampling technique thatmeasures voltages arising from within the substrate. Referring to FIG.2, the system 30 includes a mode-locked Nd:YAG laser 32 that providespicosecond-range light pulses after passage through a pulse compressor34. The compressed pulses are passed through a polarizing beam splitter36, and first and second wave plates 38 and 40 to establishpolarization. The polarized light is then directed at normal incidenceonto an integrated circuit substrate 42. The pulsed compressed beam canbe focused either onto the probed conductor itself (backside probing) oronto the ground plane beneath and adjacent to the probed conductor(front-side probing). The reflected light from the substrate is divertedby the polarizing beam splitter 36 and detected by a single point slowphoto diode detector 44. The photo diode detector is also connected to adisplay 46.

A microwave generator 48 drives the substrate 42 and is also connectedto an RF synthesizer 50, which in turn is connected to a timingstabilizer 52. The pulse output of the laser 32 is likewise connected tothe timing stabilizer 52. The output of the stabilizer 52 connects backto the laser 32 so that the frequency of the microwave generator 48locks onto a frequency that is a multiple of the laser repetition rateplus an offset. As a consequence, one may analyze the electric fieldsproduced within the integrated circuit as a result of being voltagedrive, thus providing circuit analysis of the integrated circuitoperation. In essence, the voltage of the substrate imposed by themicrowave generator 48 will change the polarization in the return signalwhich results in a detectable change at the diode detector 44.

Referring to FIGS. 3A and 3B, the locations along the incident beam aredesignated a, b, c (relative to the “down” arrow), and designated alongthe reflected beam as d, e, and f (relative to the “up” arrow), and theintensity modulated output signal is designated as g. The correspondingstates of polarization exhibited in the measurement process are shown inthe similarly lettered graphs of FIG. 3B. At location a of FIG. 3A, thepolarizing beam splitter 36 provides a linearly polarized probe beam (asshown in graph a of FIG. 3B) that is passed through the first wave plate38, which is a T/2 plate oriented at 22.5 degrees relative to theincident beam polarization, so as to yield at location b the 22.5 degreeelliptically polarized beam shown in graph b of FIG. 3B). The beam thenpasses through the second wave plate 40, which is a T/2 plate orientedat 33.75 degrees relative to the incident beam, so as to rotate the beaman additional 22.5 degrees to yield at location c the 45 degreepolarization (shown in graph c of FIG. 3B), which is at 45 degrees tothe [011] direction of the substrate 42, i.e., the cleave plane of thewafer. Similar rotations are shown for the reflected beam at thesuccessive locations d, e, and f, the resultant polarizationsrespectively being as shown in graphs d, e, and f of FIG. 3B. As shownin graph f in particular, the electro-optic effect of any voltagepresent on the substrate 42 at the spot at which the beam reflectstherefrom brings about a change in the specific polarization orientationin an amount designated in graph f of FIG. 3B as &, and that change isreflected in an amplitude change or intensity modulation in the outputsignal at location g that passes to the photo-diode 44 (as shown ingraph g of FIG. 3B). It is the measurement of & that constitutes thevoltage measurement. Among the various techniques of pre-determining thevoltage patterns to be used in testing an integrated circuit, or indeedan entire printed circuit, Springer, U.S. Pat. No. 4,625,313, describesthe use in a CPU of a ROM “kernel” in which are stored both a testprogram sequence and the testing data itself.

Since the system taught by Weingarten et al. does not include a probeproximate the circuit under test the limitations imposed by capacitiveloading of the circuit to be tested is avoided. However, the systemtaught by Weingarten et al. is limited to “point probing,” by the lens41 converging the input beam into a test point on the order of onewavelength. Unfortunately, to test an entire circuit an excessive numberof tests must be performed. In addition, it is not possible to testmultiple points simultaneously without the use of multiple systems,which may be useful in testing different portions of the circuit thatare dependant upon one another. The resulting data from the system ispresented to the user as a single amplitude measurement, i.e., theintensity of the signal produced at the photo-diode 44 depends simplyupon the degree to which the polarization of the reflected lightentering the beam splitter 36 has been rotated, so that not only are theactual phase and polarization data that derive the reflection processlost, but the precision and accuracy of the measurement becomes subjectto the linearity and other properties of the photo-diode 44 and thedisplay 46.

Various other techniques by which semiconductors may be characterized,using electromagnetic radiation of different wavelengths under differentconditions is cataloged by Palik et al. in “Nondestructive Evaluation ofSemiconductor Materials and Device,” Plenum Press, New York, 1979,chapter 7, pp. 328-390. Specifically, treatment is given of (1) infraredreflection of GaAs to obtain the optical parameters n and k and then thecarrier density N and mobility u; (2) infrared transmission in GaAs todetermine k from which is determined the wavelength dependence of freecarrier absorption; (3) infrared reflection laser (spot size) scanningof and transmission through GaAs to determine free carrier density inhomogeneity, including local mode vibrations; (4) far infrared impurityspectra; (5) infrared reflection and transmission from thin films on aGaAs substrate; microwave magnetoplasma reflection and transmission; (6)submillimeter-wave cyclotron resonance in GaAs to determinemagnetotransmission; (7) ruby laser radiation to form a waveguide in aGaAs film on a GaAs substrate, the propagation features of which arethen measured using infrared radiation; (8) infrared reflectance frommultilayers of GaAs on a GaAs substrate; (9) reflectance measurements ofgraded free carrier plasmas in both PbSnTe films on PbSnTe substratesand InAs on GaAs substrates; (10) interferometric measurements of ionimplanted layers; (11) infrared restrahlen spectra, also to determinelattice damage effects; (13) ellipsometric measurements of ion-implantedGaP; (14) determination of optical constants by internal reflectionspectroscopy; (15) laser raster scanning of semiconductor devices tomeasure photoconductivity, to track the flow of logic in a MOS shiftregister (because of current saturation, the effect of the laser lightdiffers in cells in the 0 or 1 logic state), and with a more intenselaser power level to change those logic states (i.e., to write to thecircuit); (16) laser raster scanning of semiconductor devices todetermine variations in resistivity and carrier lifetimes; (17) thermalimaging of circuits to find hot spots; (18) Raman backscattering todetermine free carrier density; (19) carrier injection to study the bandedge; (20) birefringence measurements in monolayers of GaAs and AlAs onGaAs to characterize the resultant strain; (21) photoluminescence andcathodoluminescence measurements of implanted layers and acceptor anddonor densities. With the exception of (7) above which relates towaveguide transmission, and also of (15) and (17), these techniquesrelate to the characterization of static systems. While (15) relates toa spot canning technique of the operational integrated circuit and (17)relates to hot-characterization of the device temperature.

What is desired, therefore, is a high bandwidth non-invasive testingsystem for semi-conductor materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an electro-optic sampling technique usingelectro-optic light modulator.

FIG. 2 illustrates a single point detection system.

FIGS. 3A and 3B illustrate the beams of FIG. 2.

FIG. 4 illustrates one optical system for non-destructive wave fronttesting of a device under test.

FIG. 5 illustrates another optical system for non-destructive wave fronttesting of a device under test.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present inventors came to the realization that the single pointnon-invasive probing technique of semiconductor materials could beenhanced if an area significantly greater than a wavelength of theoptical test signal could be transmitted through or reflected off of asemiconductor material. Semiconductor materials generally exhibitelectro-optic or photo-refractive effects, which can be made to becomebirefringent by the application of an electric field, either as such oras embodied in electromagnetic radiation. The present inventors thencame to the realization that if an object in a state in which it is notbirefringent, but such birefringence can then be brought about byelectrical or electromagnetic techniques, the nature of thebirefringence so introduced can be studied to determine characteristicsof the material. Upon further consideration the present inventors thencame to the realization that interferometry techniques can sense a wideregion, such as that passing through or reflected off a semiconductormaterial, which can then be analyzed.

An interference pattern is created by a coherent light beam beingtransmitted through or reflected from an object onto a recording mediumor otherwise a sensing device, which at the same time the original beamis also directed onto that recording medium or otherwise sensing deviceas a reference beam. Various characteristics of the resultanttransmitted or reflected beam, herein called the “object wave,” arerecorded in the resultant interference pattern between the object waveand the reference beam. That is to say, inasmuch as the intensities ofthe reference beam and the object wave have been recorded in thatinterference pattern, the resulting pattern typically includes a set offringes as a result of the applied voltage. Those characteristics are inpart a consequence of the physical structure (i.e., “appearance”) of theobject, hence the interference pattern is related to the structure ofthe object.

The present inventors also realized that particular semiconductormaterials are generally transparent to light of particular wavelengthsso that the light may freely pass through and reflect back though thesemiconductor, or otherwise pass through the semiconductor,substantially unaffected when the semiconductor is not stressed, such asby no applied voltage. Likewise, when the semiconductor material, suchas an integrated circuit, is stressed by applying a voltage therein byenergizing a circuit fabricated therein, the same light will reflect orotherwise pass through the semiconductor material, while being affectedby the changes imposed by the applied voltage, such as birefringence,thereby resulting in a different pattern. The stressed and unstressedstates can be recorded as different interferometry images. The twointerferometry images may then be compared to one another to determinethe actual operating characteristics within the semiconductor material.Also, two different stressed states of the semiconductor material may beobtained and thereafter two interferometry images, both from stressedstates, may be compared to one another. In addition, by its nature,interferometry techniques record a significant spatial region muchlarger than a single wavelength simultaneously which is important forcharacterizing regions of the semiconductor material. For example, theoperational characteristics of two different regions may be interrelatedwhich is unobtainable using techniques limited to a single wavelength in“spot size.” The present inventors's realization that the application ofinterferometry techniques for the testing of semiconductor devices wasonly after at least the culmination of all of the aforementionedrealizations.

Of particular interest is the “real time” characterization of operatingcharacteristics of integrated circuits where such birefringence isintroduced by the electro-optic effect, i.e., the imposition of avoltage onto the object (as in the ordinary operation of the integratedcircuit) causes birefringence therein. In other words, upon applicationof an electric field the material, such as GaAs or silicon, introducesan anisotropy and the ordinary complex refractive index n* of thematerial is decomposed into n_(o)* and n_(e)* components. Anothertechnique applicable to appropriate substrates whether or not anyoperational voltages are also applied thereto, lies in utilization ofthe photo-refraction effect, wherein electromagnetic radiation of arequired intensity is illuminated onto the substrate, and abirefringence or change in birefringence is then brought about. Inasmuchas semiconductor and like materials are generally characterized by awavelength threshold below which photo-refraction will occur, but abovewhich no photo-refraction takes place, this latter mode of operationemploys electromagnetic radiation of differing wavelengths, first tobring about a desired photo-refractive effect, and then secondly toanalyze the effect so brought about.

FIG. 4 shows an interferometry apparatus 200 comprising a laser 202 suchas a infrared DFB laser diode or the like, from which is derived a planewave of linearly polarized light 204. The optical path thus defined mayoptionally include a selected first neutral density filter 206 thatpermits convenient adjustment of the laser power level. Likewise, thebeam intensity may be varied by the applied voltage level. The beam 204from the laser 202 (or from the filter 206, if used) may then be passedinto a first broad band polarization rotator 208 for purposes of placingthe plane of polarization of the laser beam at a desired orientation.Whether or not the polarization rotator 208 is used, the beam may thenbe passed through one or more first wave plates 210 that may optionallybe used to establish a desired degree of ellipticity in the beam.Further, the wave plates may likewise establish with the beam isnon-diverging/non-converging, diverging, or converging. In any case, theresultant beam 212 is then separated into a pair of beams 214 and 216 bya beam splitter 218. The beam splitter 218 may alternatively be anydevice suitable to separate the beam 212 into multiple beams. Likewise,components or beams 214 and 216 are interchangeable.

The beam 214 may pass through a first lens 220 that will then yield anexpanded and/or expanding plane wave 222. The plane wave 222 is thenincident on a device under test 230. The plane wave 218, having awavelength suitable to pass through semiconductor material, passesthrough either the front side or the back side (or the edge) of thesurface of the device under test 230 and reflects from the interiorstructures within the device under test 230. As a result of beam 222being reflected back from the device under test 230, the reflected beamwill pass back onto beam splitter 218 so as to be passed towards andultimately impinge upon a recording device 250. The recording device 250may be any suitable type of sensing device, such as for example, acharge coupled device.

Similarly, the beam 216 may pass through a second lens 232 that willthen yield an expanded and/or expanding plane wave 234. The plane wave234 is then incident on a reflecting device 236. The plane wave 218,having a wavelength suitable to pass through semiconductor material,reflects from a reflecting device 236. As a result of beam 234 beingreflected back from the reflecting device 236, the reflected beam willbe reflected by the beam splitter 218 so as to be passed towards andultimately impinge upon the recording device 250.

Since both the reference beam (second beam 234) and the object beam(object beam 222) derive from a common, preferably coherent source(laser 202) and are simultaneously, or substantially simultaneously,incident on the recording device 250, the favorable conditions forforming an interference pattern are present. One or more of the lensesmay be omitted, as desired. Also, the object and reference beams may bereversed relative to the beam splitter, as desired. It is likewise to beunderstood that one or more light sources may be used, as desired. Also,it is to be understood that more or more recording devices may be used,as desired. In addition, it is to be understood that the recordingdevice(s) may record the object beam and the reference beamindependently of one another, which are thereafter combined in asuitable manner to generate an interference wave front pattern.

For purposes of the present invention, and in taking an initialinterference, the device under test may be any suitable device to whichthe characteristics are desired, such as for example, a functionalintegrated circuit on which the surface has been exposed (i.e., pottingis not present) but to which no voltages or other external stimuli havebeen applied, a semiconductor material such as a wafer taken from orexistent within a wafer manufacturing line, a semiconductor wafer takenfrom or existent within a chip manufacturing line at any of variousstages of manufacture (deposition, etching, metallization, etc.) or thelike, the recording device may be taken to be any suitable material forrecording or otherwise sensing an interference image, such as forexample, a photographic film, charge coupled device, or thermoplasticplate onto which the initial interference pattern is recorded in thegraphic film, charge coupled device, or thermoplastic plate onto whichthe initial interference is sensed and/or recorded.

As to the case in which the device under test is a functional but notenergized integrated circuit, a first interference may be recordedtherefrom using the apparatus as shown in FIG. 4, i.e., the interferencepattern is recorded either onto photographic film, charge coupleddevice, or within a thermoplastic plate. A second interference may thenbe made of that same to recording device while either being energizedwith a voltage or current, or illuminated with light of a wavelengthshorter than the characteristic threshold wavelength for the material.In the case in which the device under test is a semiconductor wafer, afirst interference may similarly be recorded/sensed and then a secondinterference may be recorded/sensed while illuminating the wafer in themanner just stated. In either case, any birefringence effects broughtabout either by the electro-optic effect or by the photo-refractiveeffect will then be recorded/sensed. A comparison of the twointerferences, both taken from one or the other instance of the deviceunder test, will isolate such electro-optically or photo-refractivelyproduced birefringence.

It is preferred to employ a CCD camera as the sole recording devicewhereby the first and indeed a multiplicity of subsequent interferencepatterns may be recorded, at rates commensurate with the rates ofoperation of an integrated circuit itself, i.e., 50 MHZ or more in termsof charge coupled device operation. An additional advantage in usingonly the CCD camera for recording interference is that the “reference”interference, i.e., the interference recorded from the device under test(either as an IC or as a semiconductor wafer) at a time that no voltagesor birefringence-inducing laser light was applied thereto, will berecorded digitally as well, and comparisons between the reference andsubsequent interferences may be made by means other than within theexperimental apparatus itself. i.e., by ordinary digital signalprocessing (DSP).

For the purpose of processing such a data stream an analyzer connectedto the recording device, and then a monitor connecting to analyzer.Inasmuch as the laser source in the present embodiment is preferably aDBF infrared laser diode (e.g., 900 nm-1600 nm, or 1000 nm-1500 nm, or2000 nm-14,000 nm), the data to be analyzed may be generated by means oftriggering the recording of CCD images in synchrony with the impositionof particular voltage data onto the test object, which may be an IC orpossibly an entire printed circuit. As noted previously, the Springerpatent describes the use of a digital “kernel” comprising apredetermined test program together with the digital data to be employedby that program, both of which are stored in ROM. The Springer apparatusthen uses voltage probes and the like applied to various circuit nodesto test circuit performance in a “manual” fashion; the presentinvention, of course, in addition permits an “automatic” process oftesting an entire IC, circuit board or a semiconductor wafer at anydesired stage of manufacture.

During operation a first interference pattern, stressed or unstressed,may be obtained with the “fringes” around a particular feature ofinterest identified. With changes in the applied voltage and/or fieldthe location and/or density of the fringes will vary. However, withslight changes in the fields the exact applied field and/or voltage maybe difficult at times to determine. The determination may be assisted byunderstanding the material's optical properties and physicalcharacteristics (e.g., thickness, layout, doping profile, shape, etc.).Accordingly, the reflecting device 236 may include an adjustmentmechanism to vary the location and/or angle of the reflecting device 236with respect to the beam incident thereon. By varying the position ofthe reflecting device 236 the location of the fringes may be modified,such as to line up with respect to a feature, such as a conductor.Thereafter a second interference pattern, stressed or unstressed, may beobtained with the “fringes” around a particular feature of interestidentified. The change in the fringes between the two states, togetherwith known characteristics of the particular materials within the deviceunder test in the region of interest, may be used to determine thevoltage or relative voltage change within the material in the region ofinterest. Similarly, the change in the wave front fringes between thetwo states, together with known voltages or relative voltage change, maybe used to characterize the particular materials within the device undertest in the region of interest. The change in the wave front fringes maybe determined, for example, by subtraction, by addition, or any othersuitable image comparison operation. It would likewise be noted thatmany such operations, such as subtraction, are capable of resolvingfeatures less than one wavelength in size. In addition, changes in thewave front fringes with known devices, using VLSI or VHDL circuitcoordinate maps (or the like) may be used to characterize voltages andvoltage changes. This permits for the observation of voltages withinindividual devices such as transistors or analysis of device registersor individual values of larger structures such as a micro-controller, orcharacterize fringes within the doped and non-doped conductive,semi-conductive, and non-conductive material (e.g., dielectric material)adjacent conductors, non-conductors, or semi-conductor material, or thelike. Also, this technique may be used to study the effects of incidentradiation, such as radio waves, x-rays, magnetic fields, chemicalsolutions upon the materials, etc.

It will be understood by those of ordinary skill in the art that otherarrangements and disposition of the aforesaid components, thedescriptions of which are intended to be illustrative only and notlimiting, may be made without departing from the spirit and scope of theinvention, which must be identified and determined only from thefollowing claims and equivalents thereof.

Referring to FIG. 5, another alternative design for the optical systemis illustrated for introducing an additional spatial shifting feature tothe system. In the reference beam path an spatial beam adjustment member300 is included. The spatial beam adjustment member 300 spatiallyoffsets the reflected beam relative to the incident beam. In addition,the spatial beam adjustment member 300 may likewise be adjustable to anysuitable angle. By recording the interference patterns at multipledifferent angles, for the same object beam, and processing the same aspreviously described you may obtain parallax information. In essence,this parallax information provides some three-dimensional informationwith respect to the structure and voltages within the device under test.

1. A method of testing a device under test comprising: (a) providing abeam of light from a light source having a first wavelength; (b)imposing said beam of light on a test device over a spatial regionwithin said test device substantially greater than said firstwavelength, wherein said test device has a first state of refractiveindexes; (c) imposing said beam of light on said test device over aspatial region within said test device substantially greater than saidfirst wavelength, wherein said test device has a second state ofrefractive indexes; (d) obtaining data resulting from the interferenceof said first beam and said second beam within said device under testrepresentative of the voltages within said region; (e) wherein saidfirst state of refractive indexes is at a first voltage potential, andwherein said second state of refractive indexes is at a second voltagepotential different from said first voltage potential.
 2. The method ofclaim 1 wherein said beam is provided from a laser.
 3. The method ofclaim 1 wherein said coherent light is infrared.
 4. The method of claim3 wherein said test device is silicon.
 5. The method of claim 1 whereinsaid interference of said first beam and said second beam is within saidtest device.
 6. The method of claim 1 wherein said interference of saidfirst beam and said second beam is calculated.
 7. The method of claim 1wherein said first state of birefringence is without a voltage beingapplied thereto.
 8. The method of claim 7 wherein said second state ofbirefringence is with a voltage being applied thereto.
 9. The method ofclaim 1 wherein said first state of birefringence is at a first voltagepotential.
 10. The method of claim 9 wherein said second state ofbirefringence is at a second voltage potential different from said firstvoltage potential.